Serious problems with rotating industrial equipment, such as power generating equipment, can often be avoided by monitoring various parameters of the equipment to determine whether such equipment may have an operating problem. Velomitors®1 are one brand of velocity transducers that are used to provide protection of industrial machinery by measuring equipment vibration. Velomitor® is a registered trademark of Bently Nevada.
Velomitors® are used in a number of environments. One application in which these transducers are used is the monitoring of equipment in nuclear plants where the transducers are continuously exposed over time to low levels of gamma-radiation. Velomitors® used in this type of environment are typically exposed to radiation over a long period of time. The exposed Velomitors® will function in this kind of environment for a period of time, but eventually the output bias voltage of these transducers shifts as they are exposed to the radiation over an extended period of time. Ultimately, the output of a transducer will drift close to one of its supply voltage levels, whereupon the transducer stops functioning. This problem arises when a junction field effect transistor (“JFET”) used in the Velomitor® to form a common source amplifier stage is irradiated by the gamma-radiation. The irradiation causes increasing current leakage into the JFET's gate over time. This increase in gate current, when coupled with a large feedback resistor in series with the gate, produces a high offset voltage at the transducer's output, driving the transducer's output toward the device's rail (supply voltage).